1. Field of the Invention
This invention relates to signal processing of image signals from an image sensor, and more particularly, to a method and apparatus for generating correction values to account for variations in the sensitivity of different photosites on a solid state image sensor.
2. Description Relative to the Prior Art
Variations in photodiode dark current and sensitivity as well as light source non-uniformities can cause noticeable degradation in the quality of scanned images from a solid state sensor. These variations can be compensated by calibrating the system with no illumination upon the sensor to determine an average black level, and by calibrating the system with the sensor under full illumination to determine an average maximum (white) value. It is further well known to perform a calibration with regard to each sensor photosite, as shown in the following two examples. In U.S. Pat. No. 4,602,291, the "dark current" from the imager array (i.e., the signal obtained when no light strikes the array) is directed via an analog/digital converter to an offset memory, which stores the dark current charge from each photosite in digital form. The offset value is subsequently processed with a white level value from each photosite (i.e., the signal obtained when a uniform illumination strikes the imager) and the difference thereof is stored in a gain memory for each photosite. In U.S. Pat. No. 4,760,464, a white value for each picture element is obtained by scanning a white substrate a plurality of times and storing the data obtained for each scan, and constantly replacing a previous white value by a present white value when the present value is greater than the previous level. Correction is then effected using the maximum values.
In the calibration mode described in U.S. Pat. No. 4,343,021, the sensor is presented with a field of uniform brightness. As a particular sensor element is scanned, the raw video signal is applied to a multiplier and multiplied by a correction coefficient. A comparator then decides whether the real time, processed multiplier signal is greater or less than a reference signal. A register, which temporarily stores the correction coefficient, is then either incremented or decremented and the adjusted correction coefficient value is returned to its memory location. The next time the image element is scanned, the coefficient is again drawn from memory and applied to both the multiplier and the indexing register, and the aforementioned process is repeated. After many further passes, the correction coefficient is altered in such a way that the processed video data approaches the reference signal.
Each of the aforementioned systems have certain disadvantages. The first system, disclosed in the '291 patent, calculates coefficients from only one scan. Such data may, however, contain errors caused by dust particles or surface flaws on the reference object. The second reference, the '464 patent, scans a plurality of times but skews every adjustment toward the maximum variation observed, in effect tending to clamp to the largest noise signal. The latter reference, the '021 patent, achieves a convergence toward a reference value but at the expense of a relatively complex system involving multipliers and comparators.